A passive optical network, known as a PON system, is capable of containing a predetermined number (e.g. 32 to 64) of optical network units (ONUs) using passive optical couplers. Each user can connect his/her ONU to any optical fiber (any user connection port) within the same PON system. An optical line terminal (OLT) is disposed in a central station facing to the ONUs.
Japanese Laid-Open Patent Application JP9043432 discloses a configuration in which an identification marker having a specific reflection wavelength is embedded in each optical fiber as a means to identify respective optical fibers in an optical transmission system having a plurality of branch optical fibers, each connecting to a user, like a PON system.
A voice over IP (VoIP) system has become popular. When the VoIP system becomes suitable for emergency calls as well, it would be able to completely replace the fixed telephone network. It is indispensable for an emergency call to provide a caller's location.
A conventional wired telephone is connected through a point-to-point link and, accordingly, when an emergency call is received, it is possible to determine a location where a caller's unit is installed, namely the exact location the emergency call is transmitted, through its telephone number.
In a PON system which uses a point-to-multipoint link, however, since each ONU can freely move within the same PON system as stated above, it is not possible under the present circumstances to determine a location an emergency call is transmitted. For instance, when one user takes his/her ONU to another user's house where a user connection port in the same PON system is installed and makes an emergency call, it would be misidentified as the emergency call is transmitted from a registered address (usually user's home) of the ONU.
Similar or even worse problems arise when an IP phone usable for a wireless LAN is used for an emergency call. Since a wireless LAN allows the use of other users' wireless IP phones, it is impossible to determine a caller's location from his/her account name or user ID.
In addition, there is a possibility that someone calls up pretending to be another person by secretly using his/her ONU ID and therefore it is very risky to entirely depend on the ONU ID.
When a line identification function described in the above-mentioned reference is used, it is possible to identify a location of each user connection port through searching from a central station when an optical fiber is installed. However, when an emergency call is actually dialed, this configuration is not sufficient to pinpoint a user connection port connected to the caller's IP phone. That is, in order to identify a location of an emergency call, it is necessary to pinpoint the caller's ONU first. Considering the possibility that the ONU ID might be cheated in the first place, the central station must scan its entire user connection ports to identify an optical fiber having the user connection port to which the caller's ONU is connected. This method is time-consuming and unrealistic in case of emergency.
Moreover, in the configuration described in the above reference, it is necessary to provide an optical fiber with a reflector having a specific reflection wavelength for each user and thus production costs would increase. In addition, it is required to check each optical fiber using an exclusive optical wavelength and hence it imposes a burden on an OLT in the central station.